copy reaction scheme using tikz

Question

I would like to copy the reaction scheme similar to the image provided. This is what I have attempted so far, but I am new to chemfig so I'm not sure what I'm doing. Please provide your expertise. Thanks.

\documentclass[tikz]{standalone}
\usepackage{chemfig,siunitx}
\usetikzlibrary{calc,arrows,shapes,positioning}
\DeclareSIUnit{\molar}{M}
\pgfplotsset{compat=newest} 
\pagestyle{empty}
\setcompoundsep{7em} % (not exactly) the length of the arrows

\begin{document}

\schemestart
H \arrow(--HH){<=>[$K_{HH}$]}[90] HH \+ G \arrow(--GG){<=>[$K_{GG}$]}[-90]GG
\arrow{<=>[$K_{1}$]} HG
\arrow{<=>[$K_2$]} $HG_2$
\schemestop
\end{document}

Answer 1

You can use \subscheme like this:

\documentclass{standalone}
\usepackage{chemfig}

\begin{document}

\schemestart
\subscheme{H\arrow{<=>[*{0}$K_{hd}$]}[-90]H:H}%
\hspace{-1em}+\hspace{-1em}\subscheme{G\arrow{<=>[*{0}$K_{gd}$]}[90]G:G}%
\arrow{<=>[*{0}$K_{1}$]}H:G%
\arrow{<=>[*{0}$K_{2}$]}H:G:G
\schemestop

\end{document}